Hot forming and in-situ cooling of metallic articles

ABSTRACT

Disclosed herein is a hot forming method and apparatus that can improve strength of a product via die operation. The hot forming apparatus includes a lower die on which a workpiece is placed, an upper die coupled to the lower die to press the workpiece, a holder disposed between the upper and lower dies to form an inner space of the workpiece, and a cooling unit to cool the workpiece. The hot forming method includes coupling an upper die, a lower die and a holder with a workpiece placed on the lower die, followed by pressing the workpiece to perform a hot forming operation, determining whether or not a preset time has elapsed after pressing the workpiece, and cooling the workpiece by supplying a cooling fluid around the workpiece, if the preset time has elapsed.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This application claims priority to and the benefit of Korean Patent Application No. 10-2007-0050418, filed May 23, 2007, the disclosure of which is incorporated herein by reference in its entirety.

BACKGROUND

1. Field

The present disclosure relates to hot forming of metallic articles, and more particularly, to hot forming and cooling in-situ of metallic articles.

2. Description of the Related Technology

Although various kinds of products can be produced by hot forming methods and apparatus, a torsion beam of a rear torsion beam axle suspension for a vehicle will be described herein as one example of the products produced thereby.

The torsion beam axle suspension is generally constituted by the torsion beam and trailing arms attached to opposite ends of the torsion beam, and serves to maintain a posture of the vehicle with torsion of the torsion beam with respect to a centrifugal force applied to a vehicle body upon cornering of the vehicle. Therefore, the torsion beam is required to have a high torsion and bending rigidity.

Initially, the torsion beam was formed by bending an iron plate to have a U-shaped or V-shaped cross-section, and provided therein with a torsion bar for reinforcement thereof, with the opposite ends of the torsion beam welded to the trailing arms via a separate reinforcing plate to ensure the high torsion and bending rigidity.

With such a configuration, although the torsion beam can satisfactorily meet the requirement of the high torsion and bending rigidity by the reinforced strength, it has problems in that the increased number of components, such as the torsion bar and the reinforcing plate, and the increased number of assembling and welding processes result in a significant reduction in productivity, a very high frequency of defective products caused by difficulty in tolerance management in welding, and reduction in fuel efficiency of the vehicle by an increase in weight of the final product.

The foregoing discussion is to provide general background information, and does not constitute an admission of prior art.

SUMMARY

One aspect of the invention provides a hot forming apparatus comprising: a lower die; an upper die movable relative to the lower die, wherein the lower and upper dies are configured to press a workpiece placed between the lower and upper dies; a holder configured to hold the workpiece while placed between the lower and upper dies; and a cooler configured to cool the pressed workpiece while placed between the lower and upper dies.

In the foregoing apparatus, the cooler may be configured to cool at least one of the lower die, the upper die, and the holder. The cooler may be configured to quench the workpiece so as to transform at least part of the workpiece from austenite to martensite. The cooler may comprise a cooling path provided in at least one of the lower die, the upper die and the holder so as to flow a cooling fluid therethrough and a pump configured to pump the cooling fluid to the cooling path. The cooling path may comprise a die path defined in the lower or upper die, and a holder path defined in the holder. The holder path may comprise a suction path and an exhaust path surrounding the suction path.

Further in the foregoing apparatus, the apparatus may further comprise a guide member provided between the lower and upper dies and configured to guide movement of the upper die relative to the lower die. A space may be defined between the holder, the lower die and the upper die. The cooler may be a direct cooling path having an outlet configured to discharge cooling fluid toward the space. The direct cooling path may be formed in at least one of the lower die, the upper die and the holder. The holder may comprise an insert configured to be inserted in the interior space of the workpiece. The holder may comprise a cam.

Another aspect of the invention provides a hot forming method comprising: providing an apparatus comprising an upper die, a lower die and a holder; holding a workpiece with the holder between the upper die and the lower die; moving the upper die relative to the lower die and pressing the workpiece so as to perform a hot forming operation; maintaining pressing of the workpiece for a predetermined time; and cooling in-situ the pressed workpiece with a cooling fluid.

In the foregoing method, cooling may comprise quenching the workpiece to transform at least part of the workpiece from austenite to martensite. Cooling may comprise circulating a cooling fluid through a cooling path formed in at least one of the upper die, the lower die and the holder. The holder may comprise an insert which is inserted in the interior space of the workpiece. Cooling may comprise contacting the workpiece with a cooling fluid. The method may further comprise determining that the pressed workpiece reached a predetermined temperature.

An aspect of the present invention is to provide a hot forming method and apparatus that can produce a product having a bilateral symmetry.

It is another aspect of the present invention to provide the hot forming method and apparatus that can improve the strength of the product produced by die operation.

It is yet another aspect of the present invention to provide the hot forming method and apparatus that can reduce occurrence of defective products caused by shape deformation in die operation.

In accordance with one aspect of the present invention, a hot forming apparatus comprises: a lower die on which a workpiece is placed; an upper die coupled to the lower die to press the workpiece; a holder disposed between the upper and lower dies to form an inner space of the workpiece; and a cooling unit to cool the workpiece.

Preferably, the cooling unit is provided to at least one of the lower die, the upper die, and the holder. Preferably, the cooling unit is formed in the lower die, the upper die or the holder.

The cooling unit may comprise a cooling path through which a cooling fluid is supplied to transform structure of the workpiece from austenite to martensite; a pump to supply the cooling fluid into the cooling path; and a cooling pipe connecting the cooling path to the pump. Preferably, the cooling path comprises a die path defined inside the lower and upper dies; and a holder path defined inside the holder. Preferably, the holder path comprises a suction path and an exhaust path surrounding the suction path with an inner wall of the exhaust path separated a predetermined distance from an outer wall of the suction path.

Preferably, the apparatus further comprises a guide member provided between the lower and upper dies.

Preferably, a space section is defined between the holder, the lower die and the upper die.

Preferably, the apparatus further comprises a direct cooling path to supply the cooling fluid to the space section.

Preferably, the direct cooling path is formed in at least one of the lower die, the upper die and the holder.

In accordance with another aspect of the present invention, a hot forming method comprises: coupling an upper die, a lower die and a holder with a workpiece placed on the lower die, followed by pressing the workpiece to perform a hot forming operation; determining whether or not a preset time has elapsed after pressing the workpiece; and cooling the workpiece by supplying a cooling fluid around the workpiece, if the preset time has elapsed.

The cooling step may comprise quenching the workpiece to have a predetermined temperature or less within a predetermined time to transform structure of the workpiece from austenite to martensite.

The cooling step may comprise circulating the cooling fluid into at least one of the upper die, the lower die and the holder. Preferably, the cooling step comprises circulating the cooling fluid into the lower die, the upper die or the holder. In other words, the present invention can be realized in various modifications, for example, cooling the workpiece by circulating the cooling fluid into at least one of the upper die, the lower die and the holder, and alternatively, cooling the workpiece by sequentially circulating the cooling fluid.

Preferably, the cooling step comprises supplying the cooling fluid into a path defined between the holder, the upper die and the lower die to make the cooling fluid contact the workpiece.

The method may further comprise: determining whether or not the workpiece has a predetermined temperature or less after the cooling step; and separating the upper die, the lower die and the holder to eject the workpiece, if the workpiece has the predetermined temperature or less.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspect, features and advantages of the present invention will become apparent from the following description of exemplary embodiments given in conjunction with the accompanying drawings, in which:

FIG. 1 is a perspective view of a hot forming apparatus according to one embodiment of the present invention;

FIG. 2 is a front sectional view illustrating a cooling path of the hot forming apparatus according to one embodiment of the present invention;

FIG. 3 is a side sectional view illustrating a cooling path of a hot forming apparatus according to one embodiment of the present invention;

FIG. 4 is a side sectional view illustrating a holder path of the hot forming apparatus according to one embodiment of the present invention;

FIG. 5 is a flow chart of a hot forming method according to one embodiment of the present invention; and

FIG. 6 is a perspective view of a torsion beam produced by the hot forming apparatus and method according to one embodiment of the present invention;

FIG. 7 is a cross-sectional view taken along line A-A of FIG. 6;

FIG. 8 is a cross-sectional view taken along line B-B of FIG. 6; and

FIG. 9 is a cross-sectional view taken along line C-C of FIG. 6.

DETAILED DESCRIPTION OF EMBODIMENTS

Embodiments of the present invention will be described in detail with reference to the accompanying drawings. For descriptive convenience, embodiments will be described based on a method of hot forming a rear torsion beam axle suspension for a vehicle. The drawings may be exaggerated in thickness of lines or size of components for the purpose of descriptive convenience and clarity. Furthermore, terms used herein should be defined in consideration of functions of components of embodiments of the present invention and thus can be changed according to the custom or intention of users or operators. Therefore, definition of such terms should be determined according to overall disclosures set forth herein. Herein, the words “a,” “an,” and “the” are used interchangeably with “at least one” to mean one or more of the elements being described.

A torsion beam is formed by pressing a cylindrical pipe to have a two-folded U-shaped or V-shaped cross-section at a central region of the torsion beam for ensuring the torsion rigidity and to have a

-shaped cross-section at either side thereof for ensuring the bending rigidity and a wide welding area with respect to the trailing arm. With this configuration of the torsion beam, since the torsion beam axle suspension permits reduction in the number of components and processes such as assembling and welding while ensuring sufficient torsion and bending rigidity, it is believed that it can improve the productivity and the fuel efficiency of the vehicle by weight reduction while reducing the frequency of defective products, as compared with the typical torsion beam.

In forming the torsion beam to have such different cross-sectional shapes at the different portions, since an increase in the number of processes leads to a decrease in productivity and an increase in manufacturing costs resulting in low price competitiveness, it is desirable to obtain the torsion beam with a minimal number of processes.

As a method and apparatus for producing products having the U-shaped or V-shaped cross-section by pressing the cylindrical pipe, it is known in the art to employ two sets of dies that include lower dies having different shapes, or a single set of dies that includes a single upper die and several lower dies moving to one another.

The method using the two sets of dies comprises a primary preform process and a secondary precision process, which are performed with the pipe firmly secured by a mandrel at an initial stage. The method using the single set of dies is performed by a lathe-shaped upper die and the lower dies, which are divided into a single middle-set of lower dies and two side-sets of lower dies, which move to one another along a guide plane defined between the lower dies and the upper die or between the lower dies. With these methods and apparatus, it is believed that the product having the overall U-shaped or V-shaped cross-section can be satisfactorily produced. Additionally, when applied to the product having different cross-sectional shapes at different portions as in the torsion beam of the rear torsion beam axle suspension, these methods and apparatus are believed to produce the product with a minimal number of processes.

However, when forming the product, such as the torsion beam of the torsion beam axle suspension, which has the different cross-sectional shapes formed at the different portions and continuously connected to one another, the method and apparatus as described above may have a problem in that either end or a deformable portion of the product is prone to be deformed or damaged in die operation, since it is difficult to increase the strength of the either end or the deformable portion over a predetermined value.

FIG. 1 is a perspective view of a hot forming apparatus according to one embodiment of the present invention, and FIG. 2 is a front sectional view illustrating a cooling path of the hot forming apparatus according to one embodiment of the present invention.

Referring to FIGS. 1 and 2, the hot forming apparatus according to one embodiment comprises a base 10, a lower die 30 disposed on the base 10 such that a workpiece can be placed on the lower die 30, an upper die 20 movably disposed on the base 10 and coupled to the lower die 30 for hot forming of the workpiece, holders or side cores 40 movably disposed on the base 10 and interposed between the upper die 20 and the lower die 30 to form a hollow of the workpiece, and a cooling unit 50 provided to the lower die 30, the upper die 20 and the holders 40 to cool the workpiece after hot forming. In one embodiment, the holder has an insert. In another embodiment, the holder has a cam.

The base 10 comprises a lower base 12 disposed on the floor and having the lower die 30 mounted on the lower base 12, and an upper base 14 located above the lower base 12 to be moved by a base cylinder 16 in a perpendicular direction and having the upper die 20 mounted on the upper base 14.

When the upper base 14 and the upper die 20 are lowered by driving the base cylinder 16 with a workpiece placed on the lower die 30, the upper die 20 is coupled to the lower die 30, enabling a press operation to be performed by the dies.

The upper and lower dies 20 and 30 are heated to a predetermined temperature or more for hot forming of the workpiece. A heating device (not shown) to heat the dies is well known to those skilled in the art, and detailed description thereof will be omitted hereinafter. Further, since the base cylinder 16 is a hydraulic cylinder well also known in the art, detailed description of the base cylinder and an operation thereof will be omitted hereinafter.

A guide member 32 is provided between the lower die 30 and the upper die 20 to assist in precise coupling between the upper and lower dies 20 and 30.

Further, the guide member 32 serves to ensure the workpiece is correctly placed at the center of the lower die 30, so that a completed product can have a precise bilateral symmetry.

The guide member 32 is constituted by a guide piece that has a certain size and is placed on an edge of a depression of the lower die 30 where the workpiece will be seated. The guide piece constituting the guide member 32 is coupled to an upper surface of the lower die 30 by means of typical fastening members such as bolts, rivets, etc.

The holders 40 are respectively provided to a pair of cylinders 42 disposed horizontally on an upper surface of the lower base 12.

When an arm of each cylinder 42 is protruded from the cylinder 42, the holder 40 provided to an end of the arm is interposed between the upper and lower dies 20 and 30, and when the arm is inserted into the cylinder 42, the holder 40 is moved outside a space defined between the upper and lower dies 20 and 30.

In the apparatus with such a configuration as described above, when the cylinders 42 are driven with a pipe-shaped workpiece seated on the lower die 30, the holders 40 are inserted into opposite ends of the workpiece, respectively.

Then, when the base cylinder 16 is driven, the upper die 20 is lowered and coupled to the lower die 30, followed by hot forming of the workpiece to produce a desired product, that is, a torsion beam 70.

As described above, since the die machine including the upper die 20, lower die 30, and holder 40 for manufacturing a pipe-shaped beam is well known in the art, an exploded perspective view of the upper die 20, lower die 30, and holders 40 is omitted herein.

The cooling unit 50 is formed inside at least one of the upper die 20, the lower die 30 and the holders 40. The cooling unit 50 comprises a cooling path 52 through which a cooling fluid is supplied, a pump (not shown) to supply the cooling fluid into the cooling path, 52, a cooling pipe 54 connecting the cooling path 52 to the pump, and a fluid tank (not shown) filled with the cooling fluid and connected to the cooling pipe 54. The cooling path 52 is disposed to transform the structure of the torsion beam 70 from austenite formed by the hot forming operation to martensite with the cooling fluid flowing through the cooling path 52. Preferably, the cooling path 52 is disposed in the lower die 30, the upper die 20 or the holders 40 to allow quenching of the torsion beam. More preferably, a plurality of cooling paths 52 are provided into the lower die 30, the upper die 20 or the holders 40.

The cooling path 52 comprises a die path 52 a defined inside each of the upper and lower dies 20 and 30, and a holder path 52 b defined inside each of the holders 40.

The cooling pipe 54 comprises die pipes 54 a extending from the tank to the upper and lower dies 20 and 30, and holder pipes 54 b extending from the tank to the holders 40.

The cooling pipe 54 is provided at a portion extending from the tank with a valve 56, from which the cooling pipe 54 is divided into the die pipes 54 a and the holder pipes 54 b.

With this configuration, when the pump is driven after the hot forming, the cooling fluid is supplied along the cooling path 54 defined inside the lower die 30, the upper die 20 and the holders 40, quenching the torsion beam 70 located between the lower die 30, the upper die 20 and the holders 40 to have a predetermined temperature or less.

The holder path 52 b comprises a suction path 52 c extending from one end, that is, a suction port, of the holder 40 and an exhaust path 52 d surrounding the suction path 52 c with an inner wall of the exhaust path 52 d separated a predetermined distance from an outer wall of the suction path 52 c.

Therefore, after being induced into the suction port of the holder 40, the cooling fluid flows to the other end of the holder 40 along the suction path 52 c, is discharged from the suction path 52 c, flows along a space defined between the outer wall of the suction path 52 c and the inner wall of the exhaust path 52 d, and is finally discharged outside the holder 40.

The holder 40 has a square cross-sectional outer end, which corresponds to the cylinder 42, and is gradually decreased in cross-sectional area toward an inner center thereof.

The holder 40 has a V-shaped depression of which cross-sectional area is decreased toward the inner center, so that the torsion beam 70 can be formed to have a desired shape.

FIG. 3 is a side sectional view illustrating a cooling path of a hot forming apparatus according to another embodiment of the present invention, and FIG. 4 is a side sectional view illustrating a holder path of the hot forming apparatus according to one embodiment.

Referring to FIGS. 3 and 4, the hot forming apparatus according to this embodiment is similar to the above embodiment in that this embodiment also comprises a lower die 130, an upper die 120, holders 140, and a cooling unit 150. However, the cooling unit 150 of the hot forming apparatus according to this embodiment can be differentiated from the cooling unit of the above embodiment in view of various features.

The cooling unit 150 comprises a space section 100 defined between each holder 140, the lower die 130 and the upper die 120.

According to this embodiment, the hot forming apparatus further comprises a direct cooling path 110 to supply a cooling fluid into the space section 100.

Therefore, the cooling fluid supplied along the direct cooling fluid 110 to cool a torsion beam 70 facilitates cooling of the torsion beam 70 by a direct contact with the torsion beam 70 after flowing into the space section 100.

Such a configuration and operation can facilitate transformation of the microstructure of the torsion beam 70 from austenite to martensite, allowing more effective reinforcement of the torsion beam 70.

The direct cooling path 110 may be formed in at least one of the lower die 130, the upper die 120 and the holder 140. In this embodiment, the direct cooling path 110 is shown as being formed in the holder 140.

However, the direct cooling path 110 may be formed in the upper die 120 or the lower die 130. Alternatively, the direct cooling path 11 may be formed by coupling grooves in each of the components.

In the case where the direct cooling path 110 is formed in the holder 140, preferably, the direct cooling path 110 extend from an upper surface of the holder 140 into the holder 140 and is bent to extend to the space section 100 between holder paths 152 b such that the direct cooling path 110 is communicated with the space section 100.

Next, a hot forming method with the apparatus having the configuration according to one embodiment of the present invention will be described.

FIG. 5 is a flow chart of the hot forming method according to one embodiment of the present invention, FIG. 6 is a perspective view of a torsion beam produced by the hot forming apparatus and method according to this embodiment, FIG. 7 is a cross-sectional view taken along line A-A of FIG. 6, FIG. 8 is a cross-sectional view taken along line B-B of FIG. 6, and FIG. 9 is a cross-sectional view taken along line C-C of FIG. 6.

Referring to FIG. 5 to 9, the hot forming method of this embodiment comprises coupling the upper die 20, the lower die 30 and the holders 40, with a workpiece placed on the lower die 30, followed by pressing the workpiece to produce a torsion beam 70 by a hot forming operation (hereinafter, pressing operation S10), determining whether or not a preset time has elapsed after the pressing operation S10 (hereinafter, hot forming completion determining operation S20), cooling the torsion beam 70 by supplying a cooling fluid around the torsion beam 70 (hereinafter, cooling operation S30) if it is determined in the hot forming completion determining operation S20 that the preset time has elapsed, determining whether or not the torsion beam 70 has a predetermined temperature or less after the cooling operation S30 (hereinafter, temperature determining operation S40), and separating the upper die 20, the lower die 30 and the holders 40 to eject the torsion beam 70 (hereinafter, separating operation S50) if it is determined in the temperature determining operation S40 that the torsion beam 70 has the predetermined temperature or less.

Hereinafter, the hot forming method of this embodiment will be described in detail.

First, when the cylinders 42 are driven with a pipe-shaped workpiece of metal seated on the lower die 30, the holders 40 are inserted into the opposite ends of the workpiece.

As the base cylinder 16 is driven, the upper die 20 is coupled to the lower die 30, followed by performing the pressing operation S10 to produce the torsion beam 70.

At this time, the upper die 20, lower die 30 and holders 40 are heated at 600˜900° C. for the hot forming operation.

The hot forming completion determining operation S20 is a process to measure time for which hot forming is performed, and can be performed by means of a typical time measuring device such as a timer.

If it is determined in the hot forming completion determining operation S20 that the preset time has elapsed, the pump is driven to allow a cooling fluid to be supplied into the lower die 30, the upper die 20 and the holders 40 along the cooling pipes 54 and the cooling path 52, so that the cooling operation S30 can be performed for quenching the torsion beam 70.

Additionally, after being induced into the holders 40 through the holder pipes 54 b, the cooling fluid flows to an inner end of each of the holders 40 along the suction path 52 c, is discharged from the suction path 52 c, flows along a space defined between the suction path 52 c and the exhaust path 52 d, and is finally discharged outside the holder 40.

In this manner, the torsion beam 70 is subjected to quenching to have a temperature of 100˜350° C. within about 1 second.

By such a cooling operation S30, the temperature of the torsion beam 70 is lowered below a predetermined temperature or less within a preset time to transform the structure of the torsion beam 70 from austenite to martensite, thereby improving the strength of the torsion beam 70.

At this time, the cooling fluid cools the torsion beam 70 while circulating through the lower die 30, upper die 20 and holders 40.

Alternatively, the cooling operation S30 can be performed by circulating the cooling fluid only into the lower and upper dies 30 and 20 or only into the lower die 30.

Additionally, the cooling operation S30 may further comprise direct supplying the cooling fluid to the torsion beam 70 to accelerate the cooling rate to further facilitate transformation of the workpiece to the martensite structure.

For this purpose, the hot forming method may be performed using the hot forming apparatus according to the embodiment as shown in FIGS. 3 and 4.

In this method, when the cooling step S30 is started, a cooling fluid is simultaneously supplied to the cooling path 152 and to the space section 100 along the direct cooling path 110.

Therefore, as the upper die 120, lower die 130 and holders 140 are cooled, the torsion beam 70 in contact with the upper die 120, lower die 130 and holders 140 is rapidly cooled. Here, the cooling fluid supplied into the space section 100 is brought into contact with the torsion beam 70 and accelerates cooling of the torsion beam 70, so that a more effective cooling operation can be obtained.

The temperature determining operation S40 is a process to measure time for which the cooling fluid is supplied, and can be performed using the typical time measuring device as in the hot forming completion determining operation S20. If it is determined in the temperature determining operation S40 that a preset time has elapsed, manufacture of the torsion beam 70 is completed by completing the cooling operation S30.

Then, the separating operation S50 is performed in such a fashion that the base cylinder 16 and the cylinders 42 are driven in the counterclockwise direction to separate the upper die 20 and the lower die 30 while the holders 40 are separated from the opposite ends of the torsion beam 70.

With such a configuration and operation of the apparatus as described above, the torsion beam 70 is formed to have a square-shaped cross-section at either end 74, a V-shaped cross-section at a middle region 72 with upper and lower surfaces brought into close contact with each other, and a combination of the square-shaped cross-section and a V-shaped groove at a connection region 76 between the middle region 72 and either end 74 in which the V-shaped groove of the connection region 76 is formed on an upper center thereof.

As apparent from the above description, the hot forming apparatus of one embodiment is provided with a guide member to ensure a workpiece is seated on the center of a lower die and to prevent the workpiece from being deviated from the center when the upper die is coupled to the lower die, thereby providing a torsion beam with a bilateral symmetry.

Further, for the hot forming apparatus and method of one embodiment, a cooling unit is located in the hot forming apparatus for quenching the hot formed torsion beam to facilitate transformation of the torsion beam from austenite structure, which is stable at high temperatures, into martensite structure, which is stable at low temperatures and enhances the strength of the workpiece, thereby effectively preventing deformation and damage of the torsion beam.

Moreover, according to one embodiment of the present invention, hot forming and quenching are performed in a single process, and, the hot formed product is subjected to quenching with an elapse of a preset time after the hot forming, thereby simplifying the process and reducing a frequency of defective products in die operation.

Although embodiments of the present invention has been described with reference to the accompanying drawings, these embodiments are provided for the illustrative purpose, and it will be apparent to those skilled in the art that various modifications and equivalent embodiments can be made from these embodiments disclosed herein.

Furthermore, although embodiment so the present invention has been described based on the torsion beam of the rear torsion beam axle suspension for the vehicle as one example, embodiments of the present invention can be applied to torsion beams of other applications without being limited to the vehicle. Therefore, the scope of the present invention should be limited only by the accompanying claims as follows. 

1. A hot forming apparatus comprising: a lower die; an upper die movable relative to the lower die, wherein the lower and upper dies are configured to press a workpiece placed between the lower and upper dies; a holder configured to hold the workpiece while placed between the lower and upper dies; and a cooler configured to cool the pressed workpiece while placed between the lower and upper dies.
 2. The apparatus according to claim 1, wherein the cooler is configured to cool at least one of the lower die, the upper die, and the holder.
 3. The apparatus of claim 1, wherein the cooler is configured to quench the workpiece so as to transform at least part of the workpiece from austenite to martensite.
 4. The apparatus according to claim 1, wherein the cooler comprises: a cooling path provided in at least one of the lower die, the upper die and the holder so as to flow a cooling fluid therethrough; and a pump configured to pump the cooling fluid to the cooling path.
 5. The apparatus according to claim 4, wherein the cooling path comprises a die path defined in the lower or upper die, and a holder path defined in the holder.
 6. The apparatus according to claim 5, wherein the holder path comprises a suction path and an exhaust path surrounding the suction path.
 7. The apparatus according to claim 1, further comprising a guide member provided between the lower and upper dies and configured to guide movement of the upper die relative to the lower die.
 8. The apparatus according to claim 1, wherein a space is defined between the holder, the lower die and the upper die.
 9. The apparatus according to claim 8, wherein the cooler comprises a direct cooling path having an outlet configured to discharge cooling fluid toward the space.
 10. The apparatus according to claim 9, wherein the direct cooling path is formed in at least one of the lower die, the upper die and the holder.
 11. The apparatus of claim 1, wherein the holder comprises an insert configured to be inserted in the interior space of the workpiece.
 12. The apparatus of claim 1, wherein the holder comprises a cam.
 13. A hot forming method comprising: providing an apparatus comprising an upper die, a lower die and a holder; holding a workpiece with the holder between the upper die and the lower die; moving the upper die relative to the lower die and pressing the workpiece so as to perform a hot forming operation; maintaining pressing of the workpiece for a predetermined time; and cooling in-situ the pressed workpiece with a cooling fluid.
 14. The method according to claim 13, wherein cooling comprises quenching the workpiece to transform at least part of the workpiece from austenite to martensite.
 15. The method according to claim 13, wherein cooling comprises circulating a cooling fluid through a cooling path formed in at least one of the upper die, the lower die and the holder.
 16. The method of claim 13, wherein the holder comprises an insert which is inserted in the interior space of the workpiece.
 17. The method according to claim 13, wherein cooling comprises contacting the workpiece with a cooling fluid.
 18. The method according to claim 13, further comprising determining that the pressed workpiece reached a predetermined temperature. 